Publications

2018

@article{eroglu2018,
title = {Multiplex recurrence networks},
author = {D. Eroglu and N. Marwan and M. Stebich and J. Kurths},
doi = {10.1103/PhysRevE.97.012312},
year = {2018},
date = {2018-01-25},
journal = {Physical Review E},
volume = {97},
pages = {012312},
abstract = {We have introduced a multiplex recurrence network approach by combining recurrence networks with the multiplex network approach in order to investigate multivariate time series. The potential use of this approach is demonstrated on coupled map lattices and a typical example from palaeobotany research. In both examples, topological changes in the multiplex recurrence networks allow for the detection of regime changes in their dynamics. The method goes beyond classical interpretation of pollen records by considering the vegetation as a whole and using the intrinsic similarity in the dynamics of the different regional vegetation elements. We find that the different vegetation types behave more similarly when one environmental factor acts as the dominant driving force.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

We have introduced a multiplex recurrence network approach by combining recurrence networks with the multiplex network approach in order to investigate multivariate time series. The potential use of this approach is demonstrated on coupled map lattices and a typical example from palaeobotany research. In both examples, topological changes in the multiplex recurrence networks allow for the detection of regime changes in their dynamics. The method goes beyond classical interpretation of pollen records by considering the vegetation as a whole and using the intrinsic similarity in the dynamics of the different regional vegetation elements. We find that the different vegetation types behave more similarly when one environmental factor acts as the dominant driving force.

@article{goswami2018,
title = {Abrupt transitions in time series with uncertainties},
author = {B. Goswami and N. Boers and A. Rheinwalt and N. Marwan and J. Heitzig and S. F. M. Breitenbach and J. Kurths},
url = {https://www.nature.com/articles/s41467-017-02456-6},
doi = {10.1038/s41467-017-02456-6},
year = {2018},
date = {2018-01-03},
journal = {Nature Communications},
volume = {9},
pages = {48},
abstract = {Identifying abrupt transitions is a key question in various disciplines. Existing transition detection methods, however, do not rigorously account for time series uncertainties, often neglecting them altogether or assuming them to be independent and qualitatively similar. Here, we introduce a novel approach suited to handle uncertainties by representing the time series as a time-ordered sequence of probability density functions. We show how to detect abrupt transitions in such a sequence using the community structure of networks representing probabilities of recurrence. Using our approach, we detect transitions in global stock indices related to well-known periods of politico-economic volatility. We further uncover transitions in the El Niño-Southern Oscillation which coincide with periods of phase locking with the Pacific Decadal Oscillation. Finally, we provide for the first time an 'uncertainty-aware' framework which validates the hypothesis that ice-rafting events in the North Atlantic during the Holocene were synchronous with a weakened Asian summer monsoon.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Identifying abrupt transitions is a key question in various disciplines. Existing transition detection methods, however, do not rigorously account for time series uncertainties, often neglecting them altogether or assuming them to be independent and qualitatively similar. Here, we introduce a novel approach suited to handle uncertainties by representing the time series as a time-ordered sequence of probability density functions. We show how to detect abrupt transitions in such a sequence using the community structure of networks representing probabilities of recurrence. Using our approach, we detect transitions in global stock indices related to well-known periods of politico-economic volatility. We further uncover transitions in the El Niño-Southern Oscillation which coincide with periods of phase locking with the Pacific Decadal Oscillation. Finally, we provide for the first time an 'uncertainty-aware' framework which validates the hypothesis that ice-rafting events in the North Atlantic during the Holocene were synchronous with a weakened Asian summer monsoon.

Irregular sampling is a common problem in palaeoclimate studies. We propose a method that provides regularly sampled time series and at the same time a difference filtering of the data. The differences between successive time instances are derived by a transformation costs procedure. A subsequent recurrence analysis is used to investigate regime transitions. This approach is applied on speleothem based palaeoclimate proxy data from the Indonesian-Australian monsoon region. We can clearly identify Heinrich events in the palaeoclimate as characteristic changes in the dynamics.

@article{goswami2017,
title = {Inferring interdependencies from short time series},
author = {D. Goswami and P. Schultz and B. Heinze and N. Marwan and B. Bodirsky and H. Lotze-Campen and J. Kurths},
url = {http://www.ias.ac.in/article/fulltext/conf/001/01/0051-0060},
doi = {10.29195/iascs.01.01.0021},
year = {2017},
date = {2017-12-01},
journal = {Indian Academy of Sciences Conference Series},
volume = {1},
number = {1},
pages = {51-60},
abstract = {Complex networks provide an invaluable framework for the study of interlinked dynamical systems. In many cases, such networks are constructed from observed time series by first estimating the interdependencies between pairs of datasets. However, most of the classic and state-of-the-art interdependence estimation techniques require sufficiently long time series for their successful application. In this study, we present a modification of the inner composition alignment approach (IOTA), correspondingly termed mIOTA, and review its advantages. Using two coupled auto-regressive stochastic processes, we demonstrate the discriminating power of mIOTA and show that it outperforms standard interdependence measures. We then use mIOTA to derive econo-climatic networks of interdependencies between economic indicators and climatic variability for Sub-Saharan Africa (AFR) and South Asia including India (SAS). Our analysis uncovers that crop production in AFR is strongly interdependent with the regional rainfall. While the gross domestic product (GDP) as an economic indicator in AFR is independent of climatic factors, we find that precipitation in the SAS influences the regional GDP, likely reflecting the influence of the summer monsoons. The differences in the interdependence structures between AFR and SAS reflect an underlying structural difference in their overall economies, as well as their agricultural sectors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Complex networks provide an invaluable framework for the study of interlinked dynamical systems. In many cases, such networks are constructed from observed time series by first estimating the interdependencies between pairs of datasets. However, most of the classic and state-of-the-art interdependence estimation techniques require sufficiently long time series for their successful application. In this study, we present a modification of the inner composition alignment approach (IOTA), correspondingly termed mIOTA, and review its advantages. Using two coupled auto-regressive stochastic processes, we demonstrate the discriminating power of mIOTA and show that it outperforms standard interdependence measures. We then use mIOTA to derive econo-climatic networks of interdependencies between economic indicators and climatic variability for Sub-Saharan Africa (AFR) and South Asia including India (SAS). Our analysis uncovers that crop production in AFR is strongly interdependent with the regional rainfall. While the gross domestic product (GDP) as an economic indicator in AFR is independent of climatic factors, we find that precipitation in the SAS influences the regional GDP, likely reflecting the influence of the summer monsoons. The differences in the interdependence structures between AFR and SAS reflect an underlying structural difference in their overall economies, as well as their agricultural sectors.

@article{smirnov2017b,
title = {A regime shift in the Sun-Climate connection with the end of the Medieval Climate Anomaly},
author = {D. A. Smirnov and S. F. M. Breitenbach and G. Feulner and F. A. Lechleitner and K. M. Prufer and J. U. L. Baldini and N. Marwan and J. Kurths},
url = {https://www.nature.com/articles/s41598-017-11340-8},
doi = {10.1038/s41598-017-11340-8},
year = {2017},
date = {2017-09-11},
journal = {Scientific Reports},
volume = {7},
pages = {11131},
abstract = {Understanding the influence of changes in solar activity on Earth’s climate and distinguishing it from other forcings, such as volcanic activity, remains a major challenge for palaeoclimatology. This problem is best approached by investigating how these variables influenced past climate conditions as recorded in high precision paleoclimate archives. In particular, determining if the climate system response to these forcings changes through time is critical. Here we use the Wiener-Granger causality approach along with well-established cross-correlation analysis to investigate the causal relationship between solar activity, volcanic forcing, and climate as reflected in well-established Intertropical Convergence Zone (ITCZ) rainfall proxy records from Yok Balum Cave, southern Belize. Our analysis reveals a consistent influence of volcanic activity on regional Central American climate over the last two millennia. However, the coupling between solar variability and local climate varied with time, with a regime shift around 1000–1300 CE after which the solar-climate coupling weakened considerably.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Understanding the influence of changes in solar activity on Earth’s climate and distinguishing it from other forcings, such as volcanic activity, remains a major challenge for palaeoclimatology. This problem is best approached by investigating how these variables influenced past climate conditions as recorded in high precision paleoclimate archives. In particular, determining if the climate system response to these forcings changes through time is critical. Here we use the Wiener-Granger causality approach along with well-established cross-correlation analysis to investigate the causal relationship between solar activity, volcanic forcing, and climate as reflected in well-established Intertropical Convergence Zone (ITCZ) rainfall proxy records from Yok Balum Cave, southern Belize. Our analysis reveals a consistent influence of volcanic activity on regional Central American climate over the last two millennia. However, the coupling between solar variability and local climate varied with time, with a regime shift around 1000–1300 CE after which the solar-climate coupling weakened considerably.

@article{lechleitner2017,
title = {Tropical rainfall over the last two millennia: evidence for a low-latitude hydrologic seesaw},
author = { A. Lechleitner and S. F. M. Breitenbach and K. Rehfeld and H. E. Ridley and Y. Asmerom and K. M. Prufer and N. Marwan and B. Goswami and D. J. Kennett and V. V. Aquino and V. Polyak and G. H. Haug and T. I. Eglinton and J. U. L. Baldini},
doi = {10.1038/srep45809},
year = {2017},
date = {2017-04-05},
journal = {Scientific Reports},
volume = {7},
pages = {45809},
abstract = {The presence of a low- to mid-latitude interhemispheric hydrologic seesaw is apparent over orbital and glacial-interglacial timescales, but its existence over the most recent past remains unclear. Here we investigate, based on climate proxy reconstructions from both hemispheres, the inter-hemispherical phasing of the Intertropical Convergence Zone (ITCZ) and the low- to mid-latitude teleconnections in the Northern Hemisphere over the past 2000 years. A clear feature is a persistent southward shift of the ITCZ during the Little Ice Age until the beginning of the 19th Century. Strong covariation between our new composite ITCZ-stack and North Atlantic Oscillation (NAO) records reveals a tight coupling between these two synoptic weather and climate phenomena over decadal-to-centennial timescales. This relationship becomes most apparent when comparing two precisely dated, high-resolution paleorainfall records from Belize and Scotland, indicating that the low- to mid-latitude teleconnection was also active over annual-decadal timescales. It is likely a combination of external forcing, i.e., solar and volcanic, and internal feedbacks, that drives the synchronous ITCZ and NAO shifts via energy flux perturbations in the tropics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

The presence of a low- to mid-latitude interhemispheric hydrologic seesaw is apparent over orbital and glacial-interglacial timescales, but its existence over the most recent past remains unclear. Here we investigate, based on climate proxy reconstructions from both hemispheres, the inter-hemispherical phasing of the Intertropical Convergence Zone (ITCZ) and the low- to mid-latitude teleconnections in the Northern Hemisphere over the past 2000 years. A clear feature is a persistent southward shift of the ITCZ during the Little Ice Age until the beginning of the 19th Century. Strong covariation between our new composite ITCZ-stack and North Atlantic Oscillation (NAO) records reveals a tight coupling between these two synoptic weather and climate phenomena over decadal-to-centennial timescales. This relationship becomes most apparent when comparing two precisely dated, high-resolution paleorainfall records from Belize and Scotland, indicating that the low- to mid-latitude teleconnection was also active over annual-decadal timescales. It is likely a combination of external forcing, i.e., solar and volcanic, and internal feedbacks, that drives the synchronous ITCZ and NAO shifts via energy flux perturbations in the tropics.

@article{cai2017,
title = {Holocene moisture changes in western China, Central Asia, inferred from stalagmites},
author = {Y. Cai and J. C. H. Chiang and S. F. M. Breitenbach and L. Tan and H. Cheng and R. L. Edwards and Z. An},
doi = {10.1016/j.quascirev.2016.12.014},
year = {2017},
date = {2017-02-15},
journal = {Quaternary Science Reviews},
volume = {158},
pages = {15-28},
abstract = {Central Asia lies at the convergence between the Mediterranean and Asian monsoon climates, and there is a complex interaction between the westerlies with the monsoon to form the climate of that region and its variability. The region is highly vulnerable to changes in rainfall, highlighting the need to understand the underlying controls. We present a stalagmite-based δ18O record from Kesang Cave in western China, using MC-ICP-MS U-series dating and stable isotope analysis. Stalagmite calcite δ18O largely documents changes in the δ18O of precipitation. δ18O in stalagmites was low during the early and middle Holocene (10.0–3.0 ka BP), and shifted to higher values between 3.0 and 2.0 ka BP. After 2.0 ka BP, δ18O fluctuates with distinct centennial-scale variations. Drawing from results of state-of-the-art atmospheric general circulation model simulations for the preindustrial period and 9 ka BP, we propose that changes in moisture source regions and the wetter climate both contributed to the isotopic depletion of precipitation during the early and middle Holocene. Multiple records from surrounding regions indicate a generally wetter climate during the early and mid- Holocene, supporting our interpretation on the speleothem δ18O. Changes in precipitation seasonality do not appear to be a viable explanation for the observed changes, nor increased penetration of monsoonal moisture to the study site. We speculate that the climatic regime shifted around 3.0–2.0 ka BP towards a drier climate, resulting in temperature having dominant control on precipitation δ18O. The demise of three settlements around 500AD at the margin of Tarim Basin coincided with a period of decreased precipitation and increased temperature that likely affected local water resources, underscoring the potential impact of climate on human habitation in this region.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Central Asia lies at the convergence between the Mediterranean and Asian monsoon climates, and there is a complex interaction between the westerlies with the monsoon to form the climate of that region and its variability. The region is highly vulnerable to changes in rainfall, highlighting the need to understand the underlying controls. We present a stalagmite-based δ18O record from Kesang Cave in western China, using MC-ICP-MS U-series dating and stable isotope analysis. Stalagmite calcite δ18O largely documents changes in the δ18O of precipitation. δ18O in stalagmites was low during the early and middle Holocene (10.0–3.0 ka BP), and shifted to higher values between 3.0 and 2.0 ka BP. After 2.0 ka BP, δ18O fluctuates with distinct centennial-scale variations. Drawing from results of state-of-the-art atmospheric general circulation model simulations for the preindustrial period and 9 ka BP, we propose that changes in moisture source regions and the wetter climate both contributed to the isotopic depletion of precipitation during the early and middle Holocene. Multiple records from surrounding regions indicate a generally wetter climate during the early and mid- Holocene, supporting our interpretation on the speleothem δ18O. Changes in precipitation seasonality do not appear to be a viable explanation for the observed changes, nor increased penetration of monsoonal moisture to the study site. We speculate that the climatic regime shifted around 3.0–2.0 ka BP towards a drier climate, resulting in temperature having dominant control on precipitation δ18O. The demise of three settlements around 500AD at the margin of Tarim Basin coincided with a period of decreased precipitation and increased temperature that likely affected local water resources, underscoring the potential impact of climate on human habitation in this region.

@article{riechelmann2017,
title = {Sensitivity of Bunker Cave to climatic forcings highlighted through multi-annual monitoring of rain-, soil-, and dripwaters},
author = {S. Riechelmann and A. Schröder-Ritzrau and C. Spötl and D. F. C. Riechelmann and D. K. Richter and A. Mangini and N. Frank and S. F. M. Breitenbach and A. Immenhauser},
doi = {10.1016/j.chemgeo.2016.12.015},
year = {2017},
date = {2017-01-20},
journal = {Chemical Geology},
volume = {449},
pages = {194-205},
abstract = {The last two decades have seen a considerable increase in studies using speleothems as archives of past climate variability. Caves under study are now monitored for a wide range of environmental parameters and results placed in context with speleothem data. The present study investigates trends from a seven year long monitoring of Bunker Cave, northwestern Germany, in order to assess the hydraulic response and transfer time of meteoric water from the surface to the cave. Rain-, soil-, and dripwater were collected from August 2006 to August 2013 at a monthly to bimonthly resolution and their oxygen and hydrogen isotope composition was measured. Furthermore, drip rates were quantified. Due to different drip characteristics, annual mean values were calculated for the drip rates of each drip site. Correlations of the annual mean drip rate of each site with precipitation and infiltration demonstrate that the annual infiltration, and thus the annual precipitation control the inter-annual drip-rate variability for all except one site. The hydraulic response is not delayed on an annual basis. All drip sites display identical long-term trends, which suggests a draining of a common karst reservoir over these seven years of monitoring. Correlations of soil- and dripwater monthly δ18O and δD values with atmospheric temperature data reveal water transfer times of 3 months to reach a depth of 40 cm (soilwater at site BW 2) and 4 months for 70 cm depth (soilwater at site BW 1). Finally, the water reaches the cave chambers (15 to 30 m below land surface) after ca. 2.5 years. Consequently, a temporal offset of 29 to 31 months (ca. 2.5 years) between the hydraulic response time (no time lag on annual basis) and the water transfer time (time lag of 29 to 31 months) was found, which is negligible with regard to Bunker Cave speleothems because of their slow growth rates. Here, proxies recording precipitation/infiltration and temperature are registered on a decadal scale. Variations in drip rate and thus precipitation and infiltration are recorded by δ13C and Mg/Ca ratios in speleothem calcite. Speleothem δ18O values reflect both temperature and precipitation signals due to drip rate-related fractionation processes. We document that long-term patterns in temperature and precipitation are recorded in dripwater patterns of Bunker Cave and that these are linked to the North Atlantic Oscillation (NAO).},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

The last two decades have seen a considerable increase in studies using speleothems as archives of past climate variability. Caves under study are now monitored for a wide range of environmental parameters and results placed in context with speleothem data. The present study investigates trends from a seven year long monitoring of Bunker Cave, northwestern Germany, in order to assess the hydraulic response and transfer time of meteoric water from the surface to the cave. Rain-, soil-, and dripwater were collected from August 2006 to August 2013 at a monthly to bimonthly resolution and their oxygen and hydrogen isotope composition was measured. Furthermore, drip rates were quantified. Due to different drip characteristics, annual mean values were calculated for the drip rates of each drip site. Correlations of the annual mean drip rate of each site with precipitation and infiltration demonstrate that the annual infiltration, and thus the annual precipitation control the inter-annual drip-rate variability for all except one site. The hydraulic response is not delayed on an annual basis. All drip sites display identical long-term trends, which suggests a draining of a common karst reservoir over these seven years of monitoring. Correlations of soil- and dripwater monthly δ18O and δD values with atmospheric temperature data reveal water transfer times of 3 months to reach a depth of 40 cm (soilwater at site BW 2) and 4 months for 70 cm depth (soilwater at site BW 1). Finally, the water reaches the cave chambers (15 to 30 m below land surface) after ca. 2.5 years. Consequently, a temporal offset of 29 to 31 months (ca. 2.5 years) between the hydraulic response time (no time lag on annual basis) and the water transfer time (time lag of 29 to 31 months) was found, which is negligible with regard to Bunker Cave speleothems because of their slow growth rates. Here, proxies recording precipitation/infiltration and temperature are registered on a decadal scale. Variations in drip rate and thus precipitation and infiltration are recorded by δ13C and Mg/Ca ratios in speleothem calcite. Speleothem δ18O values reflect both temperature and precipitation signals due to drip rate-related fractionation processes. We document that long-term patterns in temperature and precipitation are recorded in dripwater patterns of Bunker Cave and that these are linked to the North Atlantic Oscillation (NAO).

@article{smirnov2017,
title = {Coping with dating errors in causality estimation},
author = {D. A. Smirnov and N. Marwan and S. F. M. Breitenbach and F. Lechleitner and J. Kurths},
url = {http://quest.pik-potsdam.de/wp-content/uploads/2017/03/DatingErrors_v7.pdf},
doi = {10.1209/0295-5075/117/10004},
year = {2017},
date = {2017-01-01},
journal = {Europhysics Letter},
volume = {117},
number = {1},
pages = {10004},
abstract = {We consider the problem of estimating causal influences between observed processes from time series possibly corrupted by errors in the time variable (dating errors) which are typical in palaeoclimatology, planetary science and astrophysics. "Causality ratio" based on the Wiener-Granger causality is proposed and studied for a paradigmatic class of model systems to reveal conditions under which it correctly indicates directionality of unidirectional coupling. It is argued that in the case of a priori known directionality, the causality ratio allows a characterization of dating errors and observational noise. Finally, we apply the developed approach to palaeoclimatic data and quantify the influence of solar activity on tropical Atlantic climate dynamics over the last two millennia. A stronger solar influence in the first millennium A.D. is inferred. The results also suggest a dating error of about 20 years in the solar proxy time series over the same period.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

We consider the problem of estimating causal influences between observed processes from time series possibly corrupted by errors in the time variable (dating errors) which are typical in palaeoclimatology, planetary science and astrophysics. "Causality ratio" based on the Wiener-Granger causality is proposed and studied for a paradigmatic class of model systems to reveal conditions under which it correctly indicates directionality of unidirectional coupling. It is argued that in the case of a priori known directionality, the causality ratio allows a characterization of dating errors and observational noise. Finally, we apply the developed approach to palaeoclimatic data and quantify the influence of solar activity on tropical Atlantic climate dynamics over the last two millennia. A stronger solar influence in the first millennium A.D. is inferred. The results also suggest a dating error of about 20 years in the solar proxy time series over the same period.

@article{ritter2017,
title = {Exploring the impact of diagenesis on (isotope)geochemical and microstructural alteration features in biogenic aragonite},
author = {A.-C. Ritter and V. Mavromatis and M. Dietzel and O. Kwiecien and F. Wiethoff and E. Griesshaber and L. A. Casella and W. W. Schmahl and J. Koelen and R. D. Neuser and A. Leis and D. Buhl and A. Niedermayr and S. F. M. Breitenbach and S. M. Bernasconi and A. Immenhauser},
doi = {10.1111/sed.12356},
year = {2017},
date = {2017-01-01},
journal = {Sedimentology},
abstract = {For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre-Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18O-depleted fluids for durations between two and twenty weeks. Alteration was documented using X-ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ18O signature supports transformation via dissolution and re-precipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple-blue luminescence with orange bands. The δ18O and Δ47 signatures reveal exchange with the aqueous fluid, whereas δ13C remains unaltered in all experiments, indicating a carbonate-buffered system. Clumped isotope temperatures in high temperature experiments show compositions in broad agreement with the measured temperature. Experimentally-induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow-up experiments.},
keywords = {},
pubstate = {forthcoming},
tppubtype = {article}
}

For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre-Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18O-depleted fluids for durations between two and twenty weeks. Alteration was documented using X-ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ18O signature supports transformation via dissolution and re-precipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple-blue luminescence with orange bands. The δ18O and Δ47 signatures reveal exchange with the aqueous fluid, whereas δ13C remains unaltered in all experiments, indicating a carbonate-buffered system. Clumped isotope temperatures in high temperature experiments show compositions in broad agreement with the measured temperature. Experimentally-induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow-up experiments.

@article{Weber2017,
title = {Sr-isotope analysis of speleothems by LA-MC-ICP-MS: High temporal resolution and fast data acquisition},
author = {Michael Weber and Jasper A Wassenburg and Klaus Peter Jochum and Sebastian F M Breitenbach and Jessica Oster and Denis Scholz},
url = {http://www.sciencedirect.com/science/article/pii/S0009254117304539},
doi = {https://doi.org/10.1016/j.chemgeo.2017.08.012},
issn = {0009-2541},
year = {2017},
date = {2017-01-01},
journal = {Chemical Geology},
volume = {468},
number = {Supplement C},
pages = {63 - 74},
abstract = {Abstract Speleothems are well established climate archives. A wide array of geochemical proxies, including stable isotopes and trace elements are present within speleothems to reconstruct past climate variability. However, each proxy is influenced by multiple factors, often hampering robust interpretation. Sr isotope ratios (87Sr/86Sr) can provide useful information about water residence time and water mixing in the host rock, as they are not fractionated during calcite precipitation. Laser ablation multi-collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has rarely been used for determination of Sr isotope signatures in speleothems, as speleothems often do not possess appropriately high concentrations of Sr to facilitate this analysis. Yet the advantages of this approach include rapid data acquisition, higher spatial resolution, larger sample throughput and the absence of chemical treatment prior to analysis. We present LA-MC-ICP-MS Sr isotope data from two speleothems from Morocco (Grotte de Piste) and India (Mawmluh Cave), and we compare linescan and spot analysis ablation techniques along speleothem growth axes. The analytical uncertainty of our LA-MC-ICP-MS Sr data is comparable to studies conducted on other carbonate materials. The results of both ablation techniques are reproducible within analytical error, implying that this technique yields robust results when applied to speleothems. In addition, several comparative measurements of different carbonate reference materials (i.e. MACS-3, JCt-1, JCp-1), including tests with standard bracketing and comparison of the 87Sr/86Sr ratios with a nanosecond laser ablation system and a state-of-the-art femtosecond laser ablation system, highlight the robustness of the method.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Abstract Speleothems are well established climate archives. A wide array of geochemical proxies, including stable isotopes and trace elements are present within speleothems to reconstruct past climate variability. However, each proxy is influenced by multiple factors, often hampering robust interpretation. Sr isotope ratios (87Sr/86Sr) can provide useful information about water residence time and water mixing in the host rock, as they are not fractionated during calcite precipitation. Laser ablation multi-collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has rarely been used for determination of Sr isotope signatures in speleothems, as speleothems often do not possess appropriately high concentrations of Sr to facilitate this analysis. Yet the advantages of this approach include rapid data acquisition, higher spatial resolution, larger sample throughput and the absence of chemical treatment prior to analysis. We present LA-MC-ICP-MS Sr isotope data from two speleothems from Morocco (Grotte de Piste) and India (Mawmluh Cave), and we compare linescan and spot analysis ablation techniques along speleothem growth axes. The analytical uncertainty of our LA-MC-ICP-MS Sr data is comparable to studies conducted on other carbonate materials. The results of both ablation techniques are reproducible within analytical error, implying that this technique yields robust results when applied to speleothems. In addition, several comparative measurements of different carbonate reference materials (i.e. MACS-3, JCt-1, JCp-1), including tests with standard bracketing and comparison of the 87Sr/86Sr ratios with a nanosecond laser ablation system and a state-of-the-art femtosecond laser ablation system, highlight the robustness of the method.

The East Asian-Indonesian-Australian summer monsoon (EAIASM) links the Earth's hemispheres and provides a heat source that drives global circulation. At seasonal and inter-seasonal timescales, the summer monsoon of one hemisphere is linked via outflows from the winter monsoon of the opposing hemisphere. Long-term phase relationships between the East Asian summer monsoon (EASM) and the Indonesian-Australian summer monsoon (IASM) are poorly understood, raising questions of long-term adjustments to future greenhouse-triggered climate change and whether these changes could 'lock in' possible IASM and EASM phase relationships in a region dependent on monsoonal rainfall. Here we show that a newly developed nonlinear time series analysis technique allows confident identification of strong versus weak monsoon phases at millennial to sub-centennial timescales. We find a see-saw relationship over the last 9,000 years -- with strong and weak monsoons opposingly phased and triggered by solar variations. Our results provide insights into centennial- to millennial-scale relationships within the wider EAIASM regime.

@article{lechleitner2016,
title = {Hydrological and climatological controls on radiocarbon concentrations in a tropical stalagmite},
author = {F. A. Lechleitner and J. U. L. Baldini and S. F. M. Breitenbach and J. Fohlmeister and C. {McIntyre} and B. Goswami and R. A. Jamieson and T. S. {van der Voort} and K. Prufer and N. Marwan and B. J. Culleton and D. J. Kennett and Y. Asmerom and V. Polyak and T. I. Eglinton},
doi = {10.1016/j.gca.2016.08.039},
year = {2016},
date = {2016-01-01},
journal = {Geochimica et Cosmochimica Acta},
volume = {194},
pages = {233--252},
abstract = {Precisely-dated stalagmites are increasingly important archives for the reconstruction of terrestrial paleoclimate at very high temporal resolution. In-depth understanding of local conditions at the cave site and of the processes driving stalagmite deposition is of paramount importance for interpreting proxy signals incorporated in stalagmite carbonate. Here we present a sub-decadally resolved dead carbon fraction (DCF) record for a stalagmite from Yok Balum Cave (southern Belize). The record is coupled to parallel stable carbon isotope (δ13C) and U/Ca measurements, as well as radiocarbon (14C) measurements from soils overlying the cave system. Using a karst carbon cycle model we disentangle the importance of soil and karst processes on stalagmite DCF incorporation, revealing a dominant host rock dissolution control on total DCF. Covariation between DCF, δ13C, and U/Ca indicates that karst processes are a common driver of all three parameters, suggesting possible use of δ13C and trace element ratios to independently quantify DCF variability. A statistically significant multi-decadal lag of variable length exists between DCF and reconstructed solar activity, suggesting that solar activity influenced regional precipitation in Mesoamerica over the past 1500 years, but that the relationship was non-static. Although the precise nature of the observed lag is unclear, solar-induced changes in North Atlantic oceanic and atmospheric dynamics may play a role.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Precisely-dated stalagmites are increasingly important archives for the reconstruction of terrestrial paleoclimate at very high temporal resolution. In-depth understanding of local conditions at the cave site and of the processes driving stalagmite deposition is of paramount importance for interpreting proxy signals incorporated in stalagmite carbonate. Here we present a sub-decadally resolved dead carbon fraction (DCF) record for a stalagmite from Yok Balum Cave (southern Belize). The record is coupled to parallel stable carbon isotope (&delta;13C) and U/Ca measurements, as well as radiocarbon (14C) measurements from soils overlying the cave system. Using a karst carbon cycle model we disentangle the importance of soil and karst processes on stalagmite DCF incorporation, revealing a dominant host rock dissolution control on total DCF. Covariation between DCF, &delta;13C, and U/Ca indicates that karst processes are a common driver of all three parameters, suggesting possible use of &delta;13C and trace element ratios to independently quantify DCF variability. A statistically significant multi-decadal lag of variable length exists between DCF and reconstructed solar activity, suggesting that solar activity influenced regional precipitation in Mesoamerica over the past 1500 years, but that the relationship was non-static. Although the precise nature of the observed lag is unclear, solar-induced changes in North Atlantic oceanic and atmospheric dynamics may play a role.